feat(blocks): support heading 4/5/6 blocks

[brd6]

Description

Add concrete block models for Notion heading_4, heading_5, and heading_6 payloads.

The new classes reuse the existing generic heading hydration and serialization path, so higher heading levels now preserve rich text instead of resolving to UnsupportedBlock.

Motivation and context

Notion can return higher heading levels. Without matching block classes, those payloads fall back to UnsupportedBlock, which does not hydrate the heading property and loses rich text for consumers.

How has this been tested?

• composer test
• composer dev:lint:syntax
• composer dev:lint:style
• composer dev:analyze:all

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

PR checklist

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• I have added tests to cover my changes.